a(n) = 3*2^n + 3*n + 1.

Terms

a(0) =4a(1) =10a(2) =19a(3) =34a(4) =61a(5) =112a(6) =211a(7) =406a(8) =793a(9) =1564a(10) =3103a(11) =6178a(12) =12325a(13) =24616a(14) =49195a(15) =98350a(16) =196657a(17) =393268a(18) =786487a(19) =1572922a(20) =3145789a(21) =6291520a(22) =12582979a(23) =25165894a(24) =50331721a(25) =100663372a(26) =201326671a(27) =402653266a(28) =805306453a(29) =1610612824